1. Field of the Invention
The present invention generally relates to an illumination device for an indicating needle. More particularly, the present invention relates to an illumination device for an indicating needle that includes (a) an indicating needle that has a needle body framed in a shape of a rod and configured to point indications, and a needle base having a reflection surface configured to reflect a light entering an inside of the needle base via an entrance surface thereof toward a pointed end (or a tip) of the needle body, a width of the needle base being wider than that of the needle body, and (b) light sources that are arranged around a driving shaft of the indicating needle and each configured to emit the light that enters the inside of the indicating needle via the entrance surface thereof.
2. Description of the Related Art
Currently, combination meters for an automobile, which includes for example a speedometer, a tachometer, and a fuel gauge, have an indicating needle that has illumination features for evening/nighttime driving and/or self-luminescence features. In general, illumination functionality of such automotive meters is used to ensure and improve visibility of the meters in the nighttime as well as in the daytime. FIG. 10A illustrates a known illumination device 1 that has such illumination functionality for the indicating needle. As illustrated in FIG. 10A, the illumination device 1 has an indicating needle 2 and a plurality (three in the figure) of light sources 3.
The indicating needle 2 includes a needle body 6 and a needle base 7. The needle body 6 is formed in a rod shape and configured to point indications provided for example on a speedometer dial. The needle base 7 has an entrance surface M3 and a reflection surface M2.
The entrance surface M3 is provided on a bottom surface of the needle base 7. A light ray L emitted by the light source 3 enters an inside of the indicating needle 2 via the entrance surface M3. The reflection surface M2 is provided on an upper surface of the needle base 7. The reflection surface M2 is tapered such that a height of the needle base 7 increases toward the tip of the needle body 6. The reflection surface M2 is configured to reflect the light L coming in via the entrance surface M3 of the needle base 7 toward the tip (or a pointed end) of the needle body 6.
As illustrated in FIG. 10B, a width of the needle base 7 is wider than that of the needle body 6. The light sources 3 are arranged such that they encircles a driving shaft of the indicating needle 2 and configured to emit the light L that enters inside of the indicating needle 2 via the entrance surface M3.
The light L emitted by the light source 3 enters the inside of the indicating needle 2 via the entrance surface M3. The light L coming in via the entrance surface M3 of the indicating needle 2 is reflected off the reflection surface M2 toward the pointed end of the needle body 6. The light reflected by the reflection surface M2 is further reflected on an injection-molded layer 8 of the needle body 6 toward an upper surface of the needle body 6. The layer 8 is for example formed by hot stamping or provided with fine prisms. In this manner, the upper surface of the needle body 6 is illuminated.
Japanese Patent Application Laid-Open Publication No. 2004-294344 discloses known illumination devices of a kind that have a light-guiding member configured to guide the light L emitted by the light source 3 to the entrance surface M3 of the indicating needle 2.
Such conventional illumination devices have drawbacks. In general, the light sources 3 have a predetermined intensity distribution of its outgoing light. If the light source 3 shows lambertian or near lambertian distribution, for example in a case of a white LED, then the intensity of the outgoing light L is largest when the light is emitted along a vertical line P1 normal to the surface of the light source 3 and extending from the center of the light source 3 and the intensity gradually decreases as an angle of the outgoing light L becomes wider with respect to the vertical line P1. Also, since the driving shaft of the indicating needle 2 is encircled by a plurality of the light sources 3, the light L coming in via the entrance surface M3 into the indicating needle 2 may exhibit irregularity in luminance in a circumferential direction about the driving shaft.
Also, as illustrated in FIG. 10B, the width of the needle base 7 of the conventional illumination device 1 is wider than that of the needle body 6 due to design requirement; The needle body 6 may have to be as finely pointed as possible for external appearance quality, or a weight of the indicating needle 2 may have to be limited in view of effective operation of a driving motor that turns the indicating needle.
Accordingly, when guiding the light rays L from the light source 3 to the needle body 6, the irregularity of illumination occurs depending upon at which exact point on the reflection surface M2 the light ray L is reflected. In some regions, the traveling light L is affected by a loss caused by reflection because the light L in part goes outside of the indicating needle 2 in the course of traveling, whereas, in other regions, the traveling light L would not be seriously affected by such losses.
FIGS. 11A and 11B highlight these drawbacks. As illustrated in FIG. 11A, the light L reflected off a region A of the reflection surface M2, which is directly opposite the indicating needle 2 and is indicated by a dotted line, can be guided toward the needle body 6 without being seriously affected by the above-mentioned loss. However, as illustrated in FIG. 11B, the light reflected off regions B of the reflection surface M2, which extend on both sides of the region A and are indicated by an alternate long and short dash line, is guided toward the pointed end of the needle body 6 over multiple reflections off the both sides of the needle body 6. Understandably, larger losses will result due to the increased number of times of reflection.
As has been explained above, the light rays L emitted by the light sources 3 of the indicating needle 2 and introduced inside of the indicating needle 2 via the entrance surface M3, are subject to strong irregularities in intensity. Consequently, the amount of the light coming in on the negligibly affected region A varies when the indicating needle 2 turns. Thus, the conventional illumination device 1 is affected by variation in brightness depending upon the turning of the indicating needle 2, as long as the light sources 3 are all controlled to be at a uniform level of luminance.